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I don't know if it is the right approach, but I used something like this to solve it. I just imported the project numbers from another stack using a stack reference:

new StackReference("myOrg/myStack/dev")
        .output("projectNumber")
        .apply(value -> {
            createIamMember(value.toString(),ctx);
            return Output.of(value);
        });
new StackReference("myOrg/myStack/prod")
        .output("projectNumber")
        .apply(value -> {
            createIamMember(value.toString(),ctx);
            return Output.of(value);
        });